1. Field of the Invention
This invention relates to a method of manufacture of grinding grit granulate. Such granulates may be of corundum and/or zirconium corundum or silicon carbide bound together with a synthetic resin binding medium, preferably phenol-formaldehyde resin, with the addition of fillers with grinding action, for example cryolite and/or inert constituents, for example, chalk.
2. Summary of the Prior Art
It has been proposed in U.S. Pat. No. 2,194,472 (George H. Jackson--issued Mar. 26, 1940) to manufacture a flexible grinding or abrasive medium. The carrier or backing for the abrasive agglomerate is sprinkled with such grinding grit agglomerates each agglomerate being formed by a plurality of individual grits bonded together. During the actual manufacture of the described aggregate particles, the individual components, such as grinding grit, binding medium and filler are mixed, hardened into a compact mass, subsequently crushed and the resultant crushed pieces sieved to select the desired agglomerate size for further processing.
Apart from the substantial dust generation during such crushing and sieving, the yield of useful agglomerate following the sprinkling on a carrier of the selected individual particle sizes it is uneconomically small.
According to tests, with this manufacturing method, the useful agglomerate particles amount in the final yield to an amount below 25% of the quantity initially used.
Furthermore, in German Auslegeschrift No. 2,608,273 a sheet or strip-shaped flexible grinding tool is described, on the base of which conical grinding bodies are secured, which consist of a plurality of grinding grits, which are bound together by a binding medium together with a filler. The manufacture of conical grinding bodies from a plurality of grinding grits, binding medium and fillers is characterized in German Auslegeschrift No. 2,608,273 in that the grinding grits and fluid matrix binding medium is dispersed in an organic solvent medium phase, preferably perchloro-ethylene and is held therein is suspension, until the conical grinding bodies form, which on hardening of the matrix binding medium stabilise in their conical shape.
Apart from the fact that this method relates to the formation of synthetic resin bound conical-shaped bodies described in German Offenlegungsschrift No. 2,447,520, the manufacture of the grinding bodies prepared with cone size values which can be employed individually for sprinkling involves substantial difficulties in the technical use of the necessary quantity, so that this method has not hitherto been brought into practical use. In this respect substantial disadvantages of the method arise, in particular the environment is affected by the organic solvent medium vapours, especially during the preparation of the substantially porous grinding bodies, from which the organic solvent material must be evaporated.
It has also been proposed that grinding grit agglomerates can be obtained with the exclusion of the above-mentioned disadvantages, if a pasty, water-moistened mixture of grinding grits and binding media, preferably water-soluble phenol-formaldehyde resin is pressed with the aid of a steel blade or other scraper through a sieve web or a perforated sheet with predetermined mesh or hole width, and after drying and hardening in a heating duct the cylindrical, effectively extruded, agglomerate particles are again pressed through the sieve to form the required granular grit particles.
Such cylindrical, extruded, granulate is formed by steel blades or scrapers driven over flexible sieve webs, with the steel blades scraping over the sieve mounted around the periphery of a cylindrical rotor and extending parallel to the rotational axis. The sieve web is applied around the under part of the rotor in a semi-cicle and is tensioned so that the grinding grit mixture to be granulated can be introduced from above. The driven bladed rotor touches, during rotation, the blades on the sieve web and presses the grinding grit mixture moistened with the binding materials through the sieve web, from which the continuous cylindrical, worm like, extruded granulate falls, which can be dried and hardened in an adjacent through-flow heating duct.
This mechanical arrangement has however the disadvantage that both the hard steel blades of the rotor and also the sieve web of metal or synthetic resin filaments wear relatively rapidly as a result of the abrasive action of the grit mixture. During the dry granulation carried out with the same device for breaking and sieving it was similarly observed that high wear was caused by the dried, cylindrical, granulate particles. Furthermore it was desirable, to increase the granulate yield from about 150 kilograms per hour to the large quantities required for full scale technical manufacture and furthermore to produce, as far as possible, uniform particle size.
An object of the present invention is to provide a method and apparatus which advantageously provides for the large-scale manufacture of agglomerate, grinding grit containing particles of uniform size, without the disadvantages of smaller yields and giving rise to environmental disadvantages by release of dust or solvent medium vapours.